Such a sample cell is, e.g., known from U.S. Pat. No. 4,415,809 in which an electro-optical apparatus for measurement of fat, protein, lactose and water or solids in milk is disclosed. In this apparatus, a milk sample is pumped into an optical measurement cell and then irradiated with light beams of different wavelengths. The cell comprises a pair of flat parallel optical windows, spaced from each other for containing the sample. The optical windows and the cavity are comprised in a large metal heating block equipped with a temperature sensor, heat resistors and a temperature control circuit.
One of the disadvantages of this setup is that the temperature control loop works too slowly. This leads, for example, to overshoot. When the temperature sensor measures a too low temperature, the heat resistors heat the heating block and the temperature increases over time. When the temperature sensor signals that the desired temperature is obtained, the heat resistors are turned off. The transfer of heat from the heat resistors, through the metal heating block, to the temperature sensor does however take some time. When the temperature at the temperature sensor is right and the heater resistors are turned off, the heat transfer through the metal heating block will not stop immediately. Even when adapting the temperature control loop in order to compensate for this overshoot, it is not possible to keep the sample at the desired temperature.
In the more recent U.S. Pat. No. 6,667,808 the heater element is located closer to the sample cavity, which may reduce the delay between the heater and the sample cavity. However, the use of this single heat source in the larger metal block will always lead to fluctuating temperatures.